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This invention relates to barrel-cam internal-combustion engines (reciprocating piston internal-combustion engines where motion conversion takes place via a pair of conjugate axial cams) that are specifically improved by assuring naturally balanced operation under a greater variety of as well as simpler configurations than before.
Cost reduction has always been an area of strong interest for internal-combustion engines considering their market span and production quantities. The barrel-cam engine (a reciprocating piston internal-combustion engine where motion conversion takes place via a pair of conjugate axial cams) has been one of the attempts at cost reduction for internal-combustion engines. Prior art of barrel-cam engines claim reduction of the number of parts relative to conventional crank-type engines without sacrificing reliability, performance/efficiency, or increasing NVH (Noise and Vibrational Harshness). Though the motion conversion mechanism for barrel-cam engines uses the same number and complexity of parts as a conventional crank-type engine, the prior art anticipates reducing the components needed for valve actuation and avoiding the need for balance shafts in certain configurations.
Patent applications by those including Gresse (251,607:United Kingdom) and Herrmann (U.S. Pat. No. 2,237,621 and related subsequent patents) anticipated certain engine simplifications due to the structure of the barrel-cam engine. Specifically, they anticipated the ability to actuate cylinder valves by secondary cams directly coupled to the barrel-cam shaft (the shaft analogous to the crankshaft of conventional IC engines). This is made possible because of two reasons: Firstly, the barrel-cam shaft can always be extended to be close to the cylinder heads due to the planetary arrangement of the piston-assemblies about the barrel-cam. Secondly, and more importantly, barrel-cam engines are not restricted to a single piston-assembly reciprocation per shaft revolution. Therefore the engine can be easily designed such that the rate of rotation of the valve-actuating-cams and barrel-cam shaft arc compatible. For example, in a four cycle engine, the intake and exhaust valve groups per cylinder each lift once every four cycles, or every 2 piston-assembly oscillations. Therefore, in order for the rotation rates of the valve-actuating-cams and barrel-cam shaft to be compatible, for n defined as the # single piston-assembly oscillations defined through one rotation of the barrel-cam""s piston-assembly displacement profile, n must be divisible by 2 and the intake and exhaust cam must each have n/2 lobes. Directly coupling the valve-activation-cams to the barrel-cam shaft, as proposed, avoids the need for a rotational power transfer mechanism such as gear and belt or chain and sprocket which are needed in OHC crank-type engines. The planetary arrangement of piston-assemblies also reduces the member of cam surfaces needed since the engine can be designed such that each planetary bank of cylinders per side of the barrel-cam (across the plane normal to the barrel-cam shaft) can share the same pair of cam surfaces unlike crank-type engines where each cylinder has its own pair of cams, at minimum. Because of the unique arrangement of the pistons relative to the output shaft, the barrel-cam engine can easily accomodate axial cams or radial cams with rocker arms to actuate the valves.
A number of patents such as those by Herrmann (U.S. Pat. No. 2,237,621 and associated patents) and Trimble et al. (U.S. Pat. No. 4,090,478) set one of their objectives to have a balanced engine without balance shafts. Certain cylinder configurations of crank-type engines require balance shafts that spin at twice the crankshaft speed to balance the 2nd harmonics of vibration. Avoiding the need for balance shafts and their associated rotational power transfer mechanisms results in yet another reduction in part numbers. The series of patents by Herrmann (U.S. Pat. No. 2,237,621 and related subsequent patents) use 6 double-ended piston-assemblies in a planetary arrangement about a barrel-cam having a 4 lobe sinusoidal piston-assembly displacement profile corresponding to 2 oscillations per revolution per piston-assembly. This configuration has the piston-assemblies naturally balanced. The configuration allows partial balancing of valve-assembly forces in the direction of piston motion by generally opposing actuation of valves. Valve-assembly forces in the direction of piston motion account for a large portion of the valve-assembly forces because the angle between the valves"" direction of motion and that of the pistons is typically not very large. However cycle ordering constraints restrict opposing valves from being paired exclusively on a per piston-assembly basis and hence torque imbalances by valve-assembly actuation still exist. Trimble et al. offer an approach to have a completely balanced engine in a more general case at the expense of more parts and increased size. Trimble et al. anticipate balancing by placing along a common barrel-cam shaft both an arbitrary barrel-cam engine and its xe2x80x9cmirror imagexe2x80x9d across the plane normal to the barrel-cam shaft. This balances all piston-assembly and valve-assembly forces as well as moments. However, a pair of xe2x80x9cmirroredxe2x80x9d barrel-cam engines require that the piston-assemblies from both units along the same line of motion simultaneously move inward and outward. Therefore, the piston-assemblies, in line between both opposite units, cannot be connected. As a result, each unit requires its own barrel-cam. Consequently, for the same number of cylinders, this engine occupies more space than an engine using double-ended piston-assemblies with a single barrel-cam.
Fortunately, barrel-cam engines share the same fluid dynamics and thermodynamics as conventional crank-type engines since their combustion chambers, piston shape, and valve arrangement can be made identical to their conventional counterparts. This simplifies development since the only new considerations in design are the structural and reliability issues for the motion conversion mechanisms. Operational prototypes of barrel-cam engines have been constructed to date. Extensive development has been conducted by Dyna-Cam Engine Corp. At the time of the present invention, Dyna-Cam Engine Corp. claims to have developed their seventh generation engine based on the work by Herrmann (U.S. Pat. No. 2,237,621 and related subsequent patents). Their series of engines consisted of 12 cylinder, 6 double-ended piston-assembly, barrel-cam engines. Dyna-Cam Engine Corp. presently has a barrel-cam engine of this type in production for sale. Also Dyna-Cam Engine Corp. claims to have demonstrated a number of advantages in using a barrel-cam engine including quieter vibration-free operation and a significant reduction in number of parts compared to an equivalent crank-type internal-combustion engine. The ReJen Company is also developing a barrel-cam engine that is diesel powered with a regenerated cycle for general aviation aircraft propulsion and shipboard power generation. The ReJen company has received support from government contracts through NASA and the US Navy as well as support through partnerships with Caterpillar, Inc and Alvin Lowi and Assoc.
The 12 cylinder, 6 double-ended piston-assembly, barrel-cam engine developed by Dyna-Cam Engine Corp. is particularly useful when a high number of cylinders are needed as in marine, industrial, heavy automotive, and aviation applications. However it is not well suited for applications that are cost sensitive and historically have a low number of cylinders such as in the personal automotive market. The idea proposed by Trimble et al. to utilize two xe2x80x9cmirroredxe2x80x9d barrel-cam engine units along a common barrel-cam shaft allows the use of as few as 2 cylinders for a fully balanced engine, but this comes at the expense of increased engine size and cost from an additional barrel-cam. The object of this invention is to offer more compact and simpler barrel-cam engine configurations that are suitable for cost sensitive applications such as the personal automotive market, while maintaining the exceptional natural balance characteristics possible with barrel-cam engines. Specifically, a class of naturally balanced engines is proposed where only a single barrel-cam is needed by posing relative constraints between the number and masses of uniformly spaced piston-assemblies about the barrel-cam cylinder, the shape of the barrel-cam""s piston-assembly displacement profile, and also the valve-assemblies along with their actuation properties. This allows for choice in the number of cylinders for a naturally balanced engine. For example, these constraints show that a much simpler 4 cylinder single barrel-cam engine that is naturally balanced can be realized. Furthermore, through these constraints, naturally balanced operation is ensured for a class of barrel-cam piston-assembly displacement profiles that is more general than the sinusoidal displacement profile that has been favored in prior art for single barrel-cam engines. Relaxing the choices for barrel-cam piston-assembly displacement profiles can lend to better application specific design of barrel internal-combustion engines.
The barrel-cam engine belongs to a class of engines described as multi-cylinder internal-combustion engines where translation between piston-assembly reciprocating motion and rotational motion occurs via a pair of conjugate axial cams with dual rolling followers on the piston-assembliesxe2x80x94one rolling follower per conjugate axial cam. A pair of conjugate axial cams form a grooved or ribbed axial cam that is form-closed and is frequently referred to as a barrel-cam. Specifically, the embodiment of the motion translating portion includes a piston-assembly, a barrel, and a restrained fixture where:
The piston-assembly comprises an individual or pair of pistons attached to the end(s) of a fixture where the fixture has two independently rolling bearings that serve as followers to the conjugate axial cams described later, and also has a linear bearing segment. The linear bearings distribute part of the friction-inducing action-reaction force between the piston and combustion chamber walls to the less hostile and easy-to-lubricate environment of the linear bearing. The linear bearing serves to restrict the piston-assembly to exclusively reciprocating motion without the piston-assembly rotating about the direction of reciprocation. In other words, the linear bearing resists lateral motion of the piston-assembly relative to the direction of reciprocation and resists torque about the direction of reciprocation.
The barrel comprises a rotating cylinder with conjugate axial cam surfaces on its outer wall, where xe2x80x9cconjugatexe2x80x9d implies that the cam surfaces support followers in opposing directions of force, and such that one of the two independently rolling bearings of the piston-assembly continuously follows one of the cam surfaces and the other independently rolling bearing continuously follows the other cam surface. The conjugate nature of the cams ensures that at any point, piston-assembly to barrel or barrel to piston-assembly motion transfer can take place, while the piston-assembly remains in a continuous cycle of motion.
The restrained fixture comprises the engine block in the classical sense (that holds the reciprocating piston-assemblies and engine valve-assemblies), which additionally holds the rotating barrel-cam cylinder and has the linear bearing portions that are complementary to those of the piston-assemblies.
In multi-cylinder form, the piston-assemblies are in uniformly spaced planetary arrangement about the barrel.
The proposed balanced barrel-cam engine specifically comprises of an engine of the above described class, which under the following conditions (a) and (b), naturally minimizes NVH (Noise and Vibrational Harshness) by balancing all aggregate piston-assembly forces/torques and also by optimally balancing the majority of intake/exhaust valve-assembly forces/torques. Conditions (a) and (b) accomplish this without the need of balance shafts, opposing cylinder arrangement, or xe2x80x9cmirrorxe2x80x9d imaging of another barrel-cam engine. Given a multi-cylinder engine of the form described above, define
j=# multiple of n in harmonics of the barrel-cam""s piston-assembly displacement profile (a positive integer)
n=# single piston-assembly oscillations defined through one rotation of the barrel-cam""s piston-assembly displacement profile (a positive integer)
p=# piston-assemblies (a positive integer greater than one)
Where the barrel-cam""s piston-assembly displacement profile, in addition to the above, is further constrained to be continuously differentiable and piecewise monotonic on the 2xc3x97n segments between the consecutive pairs of TDC (top dead center) and BDC (bottom dead center).
Condition (a): Relaxed Sufficient Condition for Piston-Assembly Balancingxe2x80x94If p is not a factor of (nxc3x97jxe2x88x921), (nxc3x97j), and (nxc3x97j+1) for all j that define the barrel-cam""s piston-assembly displacement profile, then perfect balancing of aggregate piston-assembly forces and torques is achieved.
Condition (b): Relaxed Sufficient Condition for Valve-Assembly Balancingxe2x80x94If n is a multiple of 2 and if predetermined perturbations are made to the barrel-cam""s piston-assembly displacement profile (that initially exhibits piston-assembly balance through Condition (a)), the valve-assembly force imbalances can be optimally balanced in the direction of piston motion through the forces induced by piston-assembly imbalance.